An image sensor is used to convert an optical image focused on the sensor into electrical signals. The image sensor typically includes an array of light detecting elements, where each element produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image.
One very common type of image sensor is a charge coupled device (CCD). Integrated circuit chips containing a CCD image sensor have a relatively low yield and are expensive due to the specialized processing involved. The CCDs also consume a relatively large amount of power. Other well known disadvantages exist.
A much less expensive type of image sensor is formed as an integrated circuit using a CMOS process. In such a CMOS type image sensor, a photodiode or phototransistor (or other suitable device) is used as the light detecting element, where the conductivity of the element corresponds to the intensity of light impinging on the element. The variable signal thus generated by the light detecting element is an analog signal whose magnitude is approximately proportional (within a certain range) to the amount of light impinging on the element.
It is known to form these light detecting elements in a two dimensional core array which is addressable by row and column. Once a row of elements has been addressed, the analog signals from each of the light detecting elements in the row are coupled to the respective columns in the array. An analog-to-digital (A/D) converter may then be used to convert the analog signals on the columns to digital signals so as to provide only digital signals at the output of the image sensor chip.
It has also been discovered by the Applicants that, for large area pixel arrays, the analog signals generated by each light detecting element are subject to varying degrees of parasitic effects such as those caused by parasitic capacitances, inductances, and resistances. These parasitic effects are difficult to control and result in degradation of the signal-to-noise ratio of the image information.
Other undesirable features of these prior art image sensors include high complexity, severe timing restrictions, and high power dissipation.
What is needed is an inexpensive, but highly efficient, two dimensional image sensor which produces more reliable images.